Delivery of Halogens to a Subterranean Formation

ABSTRACT

Compositions and methods for treating kerogen in a subterranean formation by generating bromine and other halogens in situ in a subterranean formation. In some implementations, the generation of the bromine or halogen is delayed. This can occur, for example, by the decomposition of precursors, a chemical reaction, the encapsulation of precursors or reactants, or a combination of these approaches.

TECHNICAL FIELD

This document relates to compositions and methods of delivering halogensto a subterranean formation.

BACKGROUND

Unconventional source rock formations differ from traditional sourcerock reservoirs at least in that unconventional source rock formationsinclude the organic material, kerogen. Kerogen can account for 5-10%(10-20% vol) of the source rock formation. Kerogen is a polymer-likeintertwined organic material and is known to affect the fracturebehavior and hydraulic conductivity of a hydraulic fracture. The kerogencan alter the tensile strength of the rock and as a result, contributeto greater fracturing energy needed to propagate the fracture than informations without the kerogen material.

SUMMARY

This disclosure describes compositions and method for generatinghalogens in situ in a subterranean formation.

The following units of measure have been mentioned in this disclosure:

Unit of Measure Full form ° C. Degree Celsius M Molarity, moles/litermmol milimole mL milliliter cm centimeter psi pounds per square inch

In some implementations, a composition for treating kerogen in asubterranean formation includes a polyhalogen salt encapsulated in apolymer.

This aspect, taken alone or combinable with any other aspect, caninclude the following features. The polyhalogen salt includes an anionselected from a group consisting of Br₃ ⁻, Br₂Cl⁻, BrCl₂ ⁻, BrI₂ ⁻,Br₂I⁻, I₃ ⁻, ClI₄ ⁻, BrI₆ ⁻, ICl₂ ⁻, ICl₄ ⁻ and I₃Br₄ ⁻.

This aspect, taken alone or combinable with any other aspect, caninclude the following features. The polyhalogen salt includes an alkalimetal cation or an alkaline earth metal cation.

This aspect, taken alone or combinable with any other aspect, caninclude the following features. The polyhalogen salt includes aquaternary ammonium cation selected from a group consisting oftetramethylammonium, tetraethylammonium, tetrabutylammonium,benzyltrimethylammonium, and cetyltrimethylammonium.

This aspect, taken alone or combinable with any other aspect, caninclude the following features. The polyhalogen salt includes a cationselected from a group consisting of imidazolium, pyridinium, andpyrrolidinium.

This aspect, taken alone or combinable with any other aspect, caninclude the following features. The polyhalogen salt includes a cationwith carbon-fluorine bonds.

This aspect, taken alone or combinable with any other aspect, caninclude the following features. The cation is selected from a groupconsisting of [P(CF₃)₄]⁺, [N(CF₃)₄]⁺, orbis(tri(4-fluorophenyl)phosphine)iminium.

This aspect, taken alone or combinable with any other aspect, caninclude the following features. The polymer is a polymer matrix selectedfrom a group consisting of polyvinyl butyral polymer, vinyl acetalpolymers, butyral polymers, sytrene/-maleic anyhydride copolymers, andalkylated vinylpyrrolidone copolymers.

This aspect, taken alone or combinable with any other aspect, caninclude the following features. The polymer is an enteric coating,wherein the enteric coating is selected from a group consisting ofmethylacrylate-methacrylic acid copolymers, cellulose acetate phthalate,cellulose acetate succinate, hydroxypropylmethyl cellulose phthalate,hypromellose acetate succinate, polyvinyl acetate phthalate, shellac,cellulose acetate trimellitate, sodium alginate, and zein.

In some implementations, a method of treating kerogen in a subterraneanformation includes selecting a polyhalogen salt, encapsulating thepolyhalogen salt in a polymer, mixing the encapsulated polyhalogen saltin a fluid, and flowing the mixture of encapsulated polyhalogen salt andfluid into a location within the subterranean formation at which kerogenis present.

This aspect, taken alone or combinable with any other aspect, caninclude the following features. Mixing the encapsulated polyhalogen saltin a fluid includes mixing the polyhalogen salt in a carbondioxide-based fluid or foam.

This aspect, taken alone or combinable with any other aspect, caninclude the following features. Mixing the encapsulated polyhalogen saltin a fluid includes mixing the polyhalogen salt in an aqueous-basedfluid.

This aspect, taken alone or combinable with any other aspect, caninclude the following features. Selecting the polyhalogen salt includesselecting a salt comprising an anion from a group consisting of Br₃ ⁻,Br₂Cl⁻, BrCl₂ ⁻, BrI₂ ⁻, Br₂I⁻, I₃ ⁻, ClI₄ ⁻, BrI₆ ⁻, ICl₂ ⁻, ICl₄ ⁻,and I₃Br₄ ⁻.

This aspect, taken alone or combinable with any other aspect, caninclude the following features. Selecting the polyhalogen salt includesselecting a salt comprising a cation from a group consisting of alkalimetal cations or alkaline earth metal cations.

This aspect, taken alone or combinable with any other aspect, caninclude the following features. Selecting the polyhalogen salt includesselecting a salt that comprises a quaternary ammonium cation selectedfrom a group consisting of tetramethylammonium, tetraethylammonium,tetrabutylammonium, benzyltrimethylammonium, and cetyltrimethylammonium.

This aspect, taken alone or combinable with any other aspect, caninclude the following features. Selecting the polyhalogen salt includesselecting a salt that comprises a cation selected from a groupconsisting of imidazolium, pyridinium, and pyrrolidinium cations.

This aspect, taken alone or combinable with any other aspect, caninclude the following features. Selecting the polyhalogen salt includesselecting a salt that comprises a cation with carbon-fluorine bonds.

This aspect, taken alone or combinable with any other aspect, caninclude the following features. Selecting the polyhalogen salt includesselecting a salt that comprises a cation selected from a groupconsisting of [P(CF₃)₄]⁺, [N(CF₃)₄]⁺, andbis(tri(4-fluorophenyl)phosphine)iminium cations.

This aspect, taken alone or combinable with any other aspect, caninclude the following features. Encapsulating the salt in a polymerincludes encapsulating the salt in a polymer matrix selected from agroup consisting of polyvinyl butyral polymer, vinyl acetal polymers,butyral polymers, sytrene/-maleic anyhydride copolymers, and alkylatedvinylpyrrolidone copolymers.

This aspect, taken alone or combinable with any other aspect, caninclude the following features. Encapsulating the polyhalogen salt in apolymer includes encapsulating the salt in an enteric coating, whereinthe enteric coating is selected from a group consisting ofmethylacrylate-methacrylic acid copolymers, cellulose acetate phthalate,cellulose acetate succinate, hydroxypropylmethyl cellulose phthalate,hypromellose acetate succinate, polyvinyl acetate phthalate, shellac,cellulose acetate trimellitate, sodium alginate, and zein.

In some implementations, a composition for treating kerogen in asubterranean formation includes at least one of a bromate or chloratesalt, wherein the bromate or chlorate salt is encapsulated in a firstpolymer, and an acid.

This aspect, taken alone or combinable with any other aspect, caninclude the following features. The first polymer is a polymer matrixselected from a group consisting of polyvinyl butyral polymer, vinylacetal polymers, butyral polymers, sytrene/-maleic anhydride copolymers,and alkylated vinylpyrrolidone copolymers.

This aspect, taken alone or combinable with any other aspect, caninclude the following features. The first polymer is an enteric coatingand is selected from a group consisting of methylacrylate-methacrylicacid copolymers, cellulose acetate phthalate, cellulose acetatesuccinate, hydroxypropylmethyl cellulose phthalate, hypromellose acetatesuccinate, polyvinyl acetate phthalate, shellac, cellulose acetatetrimellitate, sodium alginate, and zein.

This aspect, taken alone or combinable with any other aspect, caninclude the following features. The acid is encapsulated in a secondpolymer, wherein the second polymer is a polymer matrix selected from agroup consisting of polyvinyl butyral polymer, vinyl acetal polymers,butyral polymers, sytrene/-maleic anhydride copolymers, and alkylatedvinylpyrrolidone copolymers.

This aspect, taken alone or combinable with any other aspect, caninclude the following features. The acid is encapsulated in a secondpolymer, wherein the second polymer is an enteric coating selected froma group consisting of methylacrylate-methacrylic acid copolymers,cellulose acetate phthalate, cellulose acetate succinate,hydroxypropylmethyl cellulose phthalate, hypromellose acetate succinate,polyvinyl acetate phthalate, shellac, cellulose acetate trimellitate,sodium alginate, and zein.

This aspect, taken alone or combinable with any other aspect, caninclude the following features. The acid is lactic acid, polylacticacid, chloracetic acid, citric acid, oxalic acid, benzoic acid, furoicacid, or aqueous hydrochloric acid.

In some implementations, a method for treating kerogen in a subterraneanformation includes encapsulating at least one of a bromate or chloratesalt in a first polymer, flowing the encapsulated bromate or chloratesalt into a subterranean formation at which kerogen is present,selecting an acid, flowing the acid into a location in the subterraneanformation at which kerogen is present, and contacting the bromate orchlorate salt with the acid.

This aspect, taken alone or combinable with any other aspect, caninclude the following features. Encapsulating at least one of a bromateor chlorate salt in a first polymer includes encapsulating the bromateor chlorate salt in a polymer matrix selected from a group consisting ofpolyvinyl butyral polymer, vinyl acetal polymers, butyral polymers,sytrene/-maleic anhydride copolymers, and alkylated vinylpyrrolidonecopolymers.

This aspect, taken alone or combinable with any other aspect, caninclude the following features. Encapsulating at least one of a bromateor chlorate salt in a first polymer comprises encapsulating the bromateor chlorate salt in an enteric coating selected from a group consistingof methylacrylate-methacrylic acid copolymers, cellulose acetatephthalate, cellulose acetate succinate, hydroxypropylmethyl cellulosephthalate, hypromellose acetate succinate, polyvinyl acetate phthalate,shellac, cellulose acetate trimellitate, sodium alginate, and zein.

This aspect, taken alone or combinable with any other aspect, caninclude the following features. Selecting the acid includes selectingthe acid from a group consisting of lactic acid, polylactic acid,chloracetic acid, citric acid, oxalic acid, benzoic acid, furoic acid,and aqueous hydrochloric acid.

This aspect, taken alone or combinable with any other aspect, caninclude the following features. The selected acid is encapsulated in asecond polymer, wherein the second polymer is a polymer matrix selectedfrom a group consisting of polyvinyl butyral polymer, vinyl acetalpolymers, butyral polymers, sytrene/-maleic anhydride copolymers, andalkylated vinylpyrrolidone copolymers.

This aspect, taken alone or combinable with any other aspect, caninclude the following features. The selected acid is encapsulated in asecond polymer, wherein the second polymer is an enteric coatingselected from a group consisting of methylacrylate-methacrylic acidcopolymers, cellulose acetate phthalate, cellulose acetate succinate,hydroxypropylmethyl cellulose phthalate, hypromellose acetate succinate,polyvinyl acetate phthalate, shellac, cellulose acetate trimellitate,sodium alginate, and zein.

This aspect, taken alone or combinable with any other aspect, caninclude the following features. The salt and the acid are flowed intothe subterranean formation simultaneously.

In some implementations, a composition for treating kerogen in asubterranean formation includes at least one of a bromate or chloratesalt, wherein the bromate or chlorate salt is encapsulated in a firstpolymer, and a reducing agent.

This aspect, taken alone or combinable with any other aspect, caninclude the following features. The reducing agent is encapsulated in asecond polymer.

This aspect, taken alone or combinable with any other aspect, caninclude the following features. The first polymer is a polyvinyl butyralpolymer, vinyl acetal polymer, butyral polymer, styrene/-maleicanhydride copolymer, or alkylated vinylpyrrolidone copolymer.

This aspect, taken alone or combinable with any other aspect, caninclude the following features. The first polymer is an enteric coating,wherein the enteric coating is methylacrylate-methacrylic acidcopolymers, cellulose acetate phthalate, cellulose acetate succinate,hydroxypropylmethyl cellulose phthalate, hypromellose acetate succinate,polyvinyl acetate phthalate, shellac, cellulose acetate trimellitate,sodium alginate, or zein.

This aspect, taken alone or combinable with any other aspect, caninclude the following features. The second polymer is a polyvinylbutyral polymer, vinyl acetal polymer, butyral polymer, styrene/-maleicanhydride copolymer, or alkylated vinylpyrrolidone copolymer.

This aspect, taken alone or combinable with any other aspect, caninclude the following features. The second polymer is an entericcoating, wherein the enteric coating is a methylacrylate-methacrylicacid copolymer, cellulose acetate phthalate, cellulose acetatesuccinate, hydroxypropylmethyl cellulose phthalate, hypromellose acetatesuccinate, polyvinyl acetate phthalate, shellac, cellulose acetatetrimellitate, sodium alginate, or zein.

This aspect, taken alone or combinable with any other aspect, caninclude the following features. The reducing agent has a standardreduction potential between that of the bromate or chlorate salt and thecorresponding halogen.

This aspect, taken alone or combinable with any other aspect, caninclude the following features. The reducing agent is at least one ofsulfur, red phosphorus, bisulfite, iodide, or iodine.

In some implementations, a method for treating kerogen in a subterraneanformation includes encapsulating at least one of a bromate or chloratesalt in a first polymer, flowing the encapsulated bromate or chloratesalt into a subterranean formation at which kerogen is present, flowinga reducing agent into the subterranean formation at which kerogen ispresent, and contacting the bromate or chlorate salt with the reducingagent.

This aspect, taken alone or combinable with any other aspect, caninclude the following features. The reducing agent is encapsulating in asecond polymer before flowing the reducing agent into the subterraneanformation at which kerogen is present.

This aspect, taken alone or combinable with any other aspect, caninclude the following features. The first polymer is a polymer matrixselected from a group that consists of polyvinyl butyral polymer, vinylacetal polymer, butyral polymers, styrene/-maleic anhydride copolymer,and alkylated vinylpyrrolidone copolymer.

This aspect, taken alone or combinable with any other aspect, caninclude the following features. The first polymer is an enteric coating,wherein the enteric coating is selected from a group that consists ofmethylacrylate-methacrylic acid copolymer, cellulose acetate phthalate,cellulose acetate succinate, hydroxypropylmethyl cellulose phthalate,hypromellose acetate succinate, polyvinyl acetate phthalate, shellac,cellulose acetate trimellitate, sodium alginate, and zein.

This aspect, taken alone or combinable with any other aspect, caninclude the following features. The second polymer is a polymer matrixselected from a group that consists of polyvinyl butyral polymer, vinylacetal polymer, butyral polymers, styrene/-maleic anhydride copolymer,and alkylated vinylpyrrolidone copolymer.

This aspect, taken alone or combinable with any other aspect, caninclude the following features. The second polymer is an entericcoating, wherein the enteric coating is selected from a group thatconsists of methylacrylate-methacrylic acid copolymer, cellulose acetatephthalate, cellulose acetate succinate, hydroxypropylmethyl cellulosephthalate, hypromellose acetate succinate, polyvinyl acetate phthalate,shellac, cellulose acetate trimellitate, sodium alginate, and zein.

This aspect, taken alone or combinable with any other aspect, caninclude the following features. The reducing agent has a standardreduction potential between that of the bromate or chlorate salt and thecorresponding halogen.

This aspect, taken alone or combinable with any other aspect, caninclude the following features. The reducing agent is selected from agroup that consists of sulfur, red phosphorus, bisulfate, iodide, andiodine.

In some implementations, a composition for treating kerogen in asubterranean formation includes a fracturing fluid, and at least oneN-halosuccinimide selected from a group consisting ofN-bromosuccinimide, N-chlorosuccinimde, and N-iodosuccinimide, whereinthe N-halosuccinimide is dissolved in the fracturing fluid, and whereinthe N-halosuccinimide is present in the fracturing fluid at aconcentration of 0.001 M to 0.10 M.

This aspect, taken alone or combinable with any other aspect, caninclude the following features. The fracturing fluid is an aqueous-basedfracturing fluid.

This aspect, taken alone or combinable with any other aspect, caninclude the following features. The fracturing fluid is a carbon-dioxidebased fluid.

In some implementations, a composition for treating kerogen in asubterranean formation includes a polymer, at least oneN-halosuccinimide selected from a group consisting ofN-bromosuccinimide, N-chlorosuccinimide, and N-iodosuccinimide, whereinthe N-halosuccinimide is encapsulated in the polymer, and carbondioxide-based fluid.

This aspect, taken alone or combinable with any other aspect, caninclude the following features. The polymer is a polymer matrix selectedfrom a group consisting of polyvinyl butyral polymer, vinyl acetalpolymer, butyral polymer, styrene/-maleic anhydride copolymer, andalkylated vinylpyrrolidone copolymer.

This aspect, taken alone or combinable with any other aspect, caninclude the following features. The polymer is an enteric coatingselected from a group consisting of methylacrylate-methacrylic acidcopolymers, cellulose acetate phthalate, cellulose acetate succinate,hydroxypropylmethyl cellulose phthalate, hypromellose acetate succinate,polyvinyl acetate phthalate, shellac, cellulose acetate trimellitate,sodium alginate, and zein.

This aspect, taken alone or combinable with any other aspect, caninclude the following features. The encapsulated N-halosuccinimide isdissolved in the carbon dioxide-based fluid at a concentration of 5 to100 pounds of encapsulated N-halosuccinimide per 1000 gallons of carbondioxide-based fluid.

In some implementations, a method for treating kerogen in a subterraneanformation includes dissolving at least one N-halosuccinimide selectedfrom a group consisting of N-bromosuccinimide, N-chlorosuccinimide, andN-iodosuccinimide in a fracturing fluid to create an N-halosuccinimidesolution, and flowing the N-halosuccinimide solution into a subterraneanformation at which kerogen is present.

This aspect, taken alone or combinable with any other aspect, caninclude the following features. The N-halosuccinimide is dissolved inthe fracturing fluid at a concentration of 0.001 M to 0.10 M.

This aspect, taken alone or combinable with any other aspect, caninclude the following features. The fracturing fluid is an aqueous-basedfracturing fluid.

This aspect, taken alone or combinable with any other aspect, caninclude the following features. The fracturing fluid is a carbondioxide-based fracturing fluid.

This aspect, taken alone or combinable with any other aspect, caninclude the following features. The N-halosuccinimide is encapsulatedbefore dissolving the N-halosuccinimide in the fracturing fluid, whereinthe encapsulated N-halosuccinimide is dissolved in the fracturing fluidat a concentration of 1 to 100 pounds of encapsulated N-halosuccinimideper 10000 gallons of fracturing fluid.

This aspect, taken alone or combinable with any other aspect, caninclude the following features. The N-halosuccinimide is encapsulated ina polymer matrix selected from a group consisting of polyvinyl butyral,vinyl acetal polymer, butyral, styrene/-maleic anhydride copolymer, andalkylated vinylprrolidone copolymer.

This aspect, taken alone or combinable with any other aspect, caninclude the following features. The N-halosuccinimide is encapsulated inan enteric coating selected from a group consisting ofmethylacrylate-methacrylic acid copolymers, cellulose acetate phthalate,cellulose acetate succinate, hydroxypropylmethyl cellulose phthalate,hypromellose acetate succinate, polyvinyl acetate phthalate, shellac,cellulose acetate trimellitate, sodium alginate, and zein.

The details of one or more implementations of the disclosure are setforth in the accompanying drawings and the description that follows.Other features, objects, and advantages of the disclosure will beapparent from the description and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1A shows the structure of a [P(CF₃)₄]⁺ ion.

FIG. 1B shows the structure of a [N(CF₃)₄]⁺ ion.

FIB. 1C shows the structure of abis(tri(4-fluorophenyl)phosphine)iminium ion.

FIG. 2 is a flow chart illustrating an example method of treatingkerogen in a subterranean formation.

FIG. 3 is a flow chart illustrating an example method of treatingkerogen in a subterranean formation.

FIG. 4 is a flow chart illustrating an example method of treatingkerogen in a subterranean formation.

FIG. 5A shows an example reaction scheme of homolytic fission ofN-bromosuccinimide.

FIG. 5B shows an example reaction scheme of heterolytic fission ofN-bromosuccinimide.

FIG. 6 is a flow chart illustrating an example method of treatingkerogen in a subterranean formation.

FIG. 7A shows an example scanning electron microscope (SEM) image of afirst shale sample before treatment with bromine.

FIG. 7B shows an example SEM image of the first shale sample aftertreatment with bromine.

FIG. 7C shows an example SEM image of a second shale sample beforetreatment with bromine.

FIG. 7D shows an example SEM image of the second shale sample aftertreatment with bromine.

FIG. 7E shows an example SEM image of a third shale sample beforetreatment with bromine.

FIG. 7F shows an example SEM image of the third shale sample aftertreatment with bromine.

FIG. 8A shows an example SEM image of a shale sample after treatmentwith bromine.

FIG. 8B shows an example SEM image of a shale sample after treatmentwith bromine.

FIG. 8C shows an example SEM image of a shale sample after treatmentwith bromine.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

Reference will now be made in detail to certain embodiments of thedisclosed subject matter, examples of which are illustrated in part inthe accompanying drawings. While the disclosed subject matter will bedescribed in conjunction with the enumerated claims, it will beunderstood that the exemplified subject matter is not intended to limitthe claims to the disclosed subject matter.

Provided in this disclosure, in part, are compositions and methods forgenerating halogens in situ in a subterranean formation. In thisdisclosure, the general term “halogen” can refer to any one of thediatomic molecules Br₂, Cl₂, I₂, or F₂, unless otherwise specified.

Bromine (Br₂) is effective at partially depolymerizing kerogen in sourcerock, which allows it to dissolve in hydrocarbons and flow out of theformation. However, bromine also reacts with metal, such as that foundin down-hole tubulars. Therefore, bromine introduced to a well borecould negatively impact the integrity of the tubulars and potentially beconsumed, which limits its application in an aqueous system.Encapsulation of bromine itself is difficult because bromine is avolatile liquid. Aside from these chemical considerations, bromine andother halogens are dangerous to handle in the field.

Provided in this disclosure are compositions and methods to generatebromine and other halogens in situ in a subterranean formation, thusminimizing damage to tubulars and other parts of the wellbore, andpreventing a halogen from prematurely reacting with materials other thankerogen. In addition, generating the bromine or other halogen in situreduces the safety considerations necessary in the field.

Generating the bromine or other halogens in situ includes, in someimplementations, a delayed generation of the bromine or halogen. Thiscan occur, for example, by the decomposition of precursors, a chemicalreaction, the encapsulation of precursors or reactants, or a combinationof these approaches.

For example, bromine and other halogens can be generated in situ by thedecomposition of polyhalogen salts in a subterranean formation.Polyhalogens include polyatomic anions that contain only halogen atoms.Polyhalogens can contain one or more than one type of halogen atom, forexample I₃ ⁻ or ICl₂ ⁻.

In some implementations, salts containing polyhalogen ions areencapsulated in a polymer matrix. Polyhalogen salts can be formed by thereaction of halide ions with halogens and heteroatomic congeners to formstable polyhalogen ions. Examples of halogens include F₂, Cl₂, Br₂, andI₂. Examples of halide ions include Cl⁻, Br⁻, and I⁻. Equations 1-6 showexamples of the equilibria between halides and halogens and thecorresponding polyhalogen ion. The bonding in these polyhalogen ions isrelatively weak π-bonding. At temperatures of 130-180° C., thesepolyhalogen compounds will decompose to yield a halogen and a halide.This decomposition process is accelerated in the presence of water.Therefore, in a subterranean formation where temperatures aresufficiently high or water is present, the polyhalogen ions willdecompose to yield halide ions and halogens.

Br⁻+Cl₂↔[Cl₂Br]⁻  (Eq. 1)

Br⁻+Br₂↔[Br₃]⁻  (Eq. 2)

Br⁻+I₂↔[BrI₂]⁻  (Eq. 3)

Br⁻+3I₂↔[BrI₆]⁻  (Eq. 4)

I⁻+I₂↔[I₃]⁻  (Eq. 5)

I⁻+2I₂↔[I₅]⁻  (Eq. 6)

In some implementations, the polyhalogen salt will contain an anionselected from Br₃ ⁻, Br₂Cl⁻, BrCl₂ ⁻, BrI₂ ⁻, Br₂I⁻, I₃ ⁻, ClI₄ ⁻, BrI₆⁻, ICl₂ ⁻, ICl₄ ⁻, I₃Br₄ ⁻ and mixtures thereof. The cation in thesesalts can be an alkali cation or an alkali earth cation, for exampleLi⁺, Na⁺, K⁺, Mg²⁺, or Ca²⁺. In some implementations, the cation caninclude a quaternary ammonium cation selected from the group containingtetramethylammonium, tetraethylammonium, tetrabutylammonium,benzyltrimethylammonium, and cetyltrimethylammonium. In someembodiments, the cation can include a quaternary phosphonium cationselected from tetraphenylphosphonium, tetraethylphosphonium, andtetramethylphosphonium. In some implementations, the cation can includean imidazole, for example imidazolium ([Im]⁺), pyridinium ([Pyr]⁺),pyrrolidinium ([Pyrr]⁺), or 1-butyl-3-methyl-imidazolium ([BMIm]⁺).

In some implementations, the cation can include fluorine-carbon bonds.Carbon-fluorine bonds reduce the reactivity of a cation and increase thesolubility of a salt in organic solvents, for example in carbon dioxide.Cations including carbon-fluorine bonds can include [P(CF₃)₄]⁺ (FIG.1A), [N(CF₃)₄]⁺ (FIG. 1B), or bis(tri(4-fluorophenyl)phosphine)iminium(FIG. 1C).

In some implementations, a polyhalogen salt can be dissolved into afluid medium or fracturing fluid that includes carbon dioxide. Thisfluid can then be used in subterranean formations for hydraulicfracturing operations, and to treat kerogen in the subterraneanformation. Once in the subterranean formation, the polyhalogen saltdecomposes to yield a halogen, for example bromine, chlorine, or iodine.The halogens react with and degrade or partially degrade kerogen presentin the subterranean formation. The degradation of kerogen can improvethe hydraulic conductivity of a hydraulic fracture and increasehydrocarbon recovery from the well.

In some implementations, the polyhalogen salt can be encapsulated byincorporation into a polymer. Encapsulation into a polymer can includeincorporation or infusion into a polymer matrix. Encapsulation into apolymer can also include being surrounded by an enteric coating.

The polymer may include a polymer matrix, for example polyvinyl butyralpolymer, vinyl acetal polymers, butyral polymers, styrene/-maleicanhydride copolymers, or alkylated vinylpyrrolidone copolymers.

In some implementations, the polyhalogen salt can be encapsulated withinan enteric coating, for example with methylacrylate-methacrylic acidcopolymers, cellulose acetate phthalate, cellulose acetate succinate,hydroxypropylmethyl cellulose phthalate, hypromellose acetate succinate,polyvinyl acetate phthalate, shellac, cellulose acetate trimellitate,sodium alginate, or zein.

In some implementations, the encapsulated polyhalogen salt can be mixedinto an aqueous or carbon dioxide based fluid or foam. The fluid or foamcan then be flowed into the subterranean formation for use in hydraulicfracturing operations.

FIG. 2 is an example of a method 200 for treating kerogen in asubterranean zone. At 202, a polyhalogen salt is selected. At 204, thepolyhalogen salt is encapsulated in a polymer. At 206, the encapsulatedpolyhalogen salt is mixed in a fluid. At 208, the mixture ofencapsulated polyhalogen salt and fluid is flowed into a location withina subterranean formation at which kerogen is present. In someimplementations, the fluid is an aqueous-based fluid. In someimplementations, the fluid is a carbon dioxide-based fluid or foam.

Another approach to generate halogens in situ includes the use of anunstable oxo-acid. Unstable oxo-acids include bromic acid (HBrO₃) andchloric acid (HClO₃). HBrO₃ is a strong acid with a pKa of approximately−2, and decomposes to yield bromine. HClO₃ is a strong acid with a pKaof approximately −1, and decomposes to yield chlorine and chlorinedioxide.

Both HBrO₃ and HClO₃ can be formed by protonation of bromate orchlorate, respectively, as shown in Eq. 7, where X is Br or Cl.

$\begin{matrix}{\left\lbrack {XO}_{3} \right\rbrack \overset{- H^{+}}{\rightarrow}{HXO}_{3}} & \left( {{Eq}.\mspace{11mu} 7} \right)\end{matrix}$

Accordingly, the unstable oxo-acids can be produced in situ byprotonating bromate or chlorate anions with a hydrogen ion. The sourceof the hydrogen ion H⁺ can be an acid. The acid can be an encapsulatedsolid, or a free acid in the medium. The bromate and chlorate ions canbe present as bromate and chlorate salts with alkali cations, forexample Na⁺ and K⁺.

In some implementations, a composition for treating kerogen in asubterranean formation includes at least one bromate or chlorate salt.For example, the composition can include bromate salts, chlorate salts,or a mixture of bromate and chlorate salts. These salts or mixtures ofsalts can be encapsulated in a first polymer. The first polymer can be apolymer matrix, for example polyvinyl butyral polymer, vinyl acetalpolymers, butyral polymers, styrene/-maleic anhydride copolymers, oralkylated vinylpyrrolidone copolymers.

The first polymer can be an enteric coating, for examplemethylacrylate-methacrylic acid copolymers, cellulose acetate phthalate,cellulose acetate succinate, hydroxypropylmethyl cellulose phthalate,hypromellose acetate succinate, polyvinyl acetate phthalate, shellac,cellulose acetate trimellitate, sodium alginate, or zein. Theencapsulated bromate or chlorate salt, or mixture of encapsulatedbromate and chlorate salt, can be dispersed or suspended in a fluidmedium or fracturing fluid. The fracturing fluid can be an aqueoussolvent. Alternatively, the fracturing fluid can be a carbon dioxidebased fluid or foam.

The composition can also include a hydrogen ion source, for example, anacid. Suitable acids include lactic acid, polylactic acid, chloraceticacid, citric acid, oxalic acid, benzoic acid, furoic acid, or othersolid organic acids. Alternatively, the acid can be an inorganic acid,for example hydrochloric acid.

The acid can be encapsulated in a second polymer. The second polymer canbe a polymer matrix, for example polyvinyl butyral polymer, vinyl acetalpolymers, butyral polymers, styrene/-maleic anhydride copolymers, oralkylated vinylpyrrolidone copolymers. The second polymer can be anenteric coating, for example methylacrylate-methacrylic acid copolymers,cellulose acetate phthalate, cellulose acetate succinate,hydroxypropylmethyl cellulose phthalate, hypromellose acetate succinatepolyvinyl acetate phthalate, shellac, cellulose acetate trimellitate,sodium alginate, or zein.

The acid or encapsulated acid can be dissolved in a fluid medium orfracturing fluid to be used in treating kerogen. The fracturing fluidcan be an aqueous solution. Alternatively, the fracturing fluid can be acarbon dioxide based fluid or foam.

The encapsulated salt or mixture of encapsulated salts can be flowedinto a subterranean formation where kerogen is present. After the saltor mixture of salts has been flowed into the formation, an acid can beflowed into the formation. The polymer matrix or enteric coatingdegrades in the subterranean formation, and the salts and the acid thencome into contact. The protonation of the bromate or chlorate anionresults in the formation of an unstable oxo-acids. These oxo-acids thendecompose to form their respective halogens, for example bromine fromHBrO₃ and chlorine from HClO₃.

In some implementations, the acid can be flowed into a subterraneanformation with the same treatment volume as the salt or mixture ofsalts. In some implementations, the acid can be flowed into asubterranean formation with a different treatment volume than the saltor mixture of salts. In some implementations, the salt or mixture ofsalts and the acid are flowed into the subterranean formationsimultaneously.

In the above described compositions and methods, the encapsulation ofone or more polyhalogen salts results in the delayed generation ofhalogen. The halogen is generated in the subterranean formation thatcontains kerogen, which results in increased safety and prevents thehalogen from reacting prematurely, for example by degrading hydraulicfracturing equipment.

FIG. 3 is an example of a method 300 for treating kerogen in asubterranean zone. At 302, at least one of a bromate or chlorate salt isencapsulated in a first polymer. At 304, the encapsulated bromate orchlorate salt is flowed into a subterranean formation at which kerogenis present. At 306, an acid is selected. At 308, the acid is flowed intothe subterranean formation at which kerogen is present. At 310, thebromate or chlorate salt is contacted with the acid.

Another approach for generating halogens in situ is the reduction ofchlorate, bromate, or iodate salts. When reduced, salts that containchlorate, bromate, or iodate anions generate bromine, chlorine, andiodine, respectively (Eq. 8-10).

ClO₃ ⁻+Red_(i)→Cl₂+Red_(f)  (Eq. 8)

BrO₃ ⁻+Red_(i)→Br₂+Red_(f)  (Eq. 9)

IO₃ ⁻+Red_(i)→I₂Red_(f)  (Eq. 10)

Equations 11 and 12 schematically illustrate how a bromate, chlorate, oriodate anion (XO₃ ⁻) can react with an initial reducing agent (Red_(i))to produce a halogen (X₂) and a subsequent reducing agent (Red_(f)),where k₁ represents the rate constant for the reaction. In somecircumstances, Red_(i) can react with the generated halogen (X₂) togenerate a halide ion (X⁻), where k₂ is the rate constant for thereaction.

$\begin{matrix}{{{XO}_{3}^{-} + {Red_{i}}}\overset{k\; 1}{\rightarrow}{X_{2} + {Red_{f}}}} & \left( {{Eq}.\mspace{11mu} 11} \right) \\{{X_{2} + {Red_{i}}}\overset{k\; 2}{\rightarrow}{X^{-} + {Red_{f}}}} & \left( {{Eq}.\mspace{11mu} 12} \right)\end{matrix}$

In situations where k₂ is much less than k₁, equation 11 will occur at afaster rate than equation 12, meaning X₂ is not substantially consumedby Red_(i). In this situation, the majority of X₂ is available forreaction with kerogen. Inversely, if k₂ is greater than k₁, the halogenX₂ may be consumed by Red_(i) before it reacts with kerogen. Thereforereducing agents with a faster rate of reaction with oxo-anions (XO₃ ⁻)relative to halogens (X₂) are suitable for generating halogens to treatkerogen.

Another approach for the efficient reduction of oxo-anions is to use areducing agent with a standard oxidation potential (Q) between that ofthe halogen and the oxo-anion. This ensures that the reduction ofoxo-anions is more favorable than the reduction of halogens.

For example, the reaction between bromate and a reducing agent Red_(i)can be expressed as two half reactions:

2[BrO₃]⁻+12H⁺+10e ⁻→Br₂+6H₂O E_(red)=1.48V  (Eq. 13)

Red_(i)→Red_(f)+2e ⁻ E_(ox)=Q  (Eq. 14)

where E_(red) and E_(ox) are the standard reduction and oxidationpotentials of the respective half reactions.

Similarly, the reaction between bromine and a reducing agent Red_(i) canbe expressed as two half reactions:

Br₂+2e ⁻→2Br⁻ E_(red)=1.08V  (Eq. 15)

Red_(i)→Red_(f)+2e ⁻ E_(ox)=Q  (Eq. 16)

where E_(red) and E_(ox) are the standard reduction and oxidationpotentials of the respective half reactions.

The difference in the reduction and oxidation potentials(E_(red)−E_(ox)) indicates whether the overall oxidation-reductionreaction is thermodynamically favorable. If the difference in reductionand oxidation potentials is positive, the reaction is favorable.

Accordingly, if Q is between 1.48V and 1.08V, the reduction of bromatewill be thermodynamically favorable whereas the reduction of Br₂ willnot be thermodynamically favorable.

As an illustration, if the reducing agent Red_(i) has an oxidationpotential Q of 1.2V, the difference in reduction and oxidationpotentials between the bromate anion and the reducing agent will be1.48V−1.2V, or 0.28V. The positive difference indicates that thereaction is thermodynamically favorable. However, the difference inreduction and oxidation potentials between the halogen Bra and thereducing agent will be 1.08V−1.2V, or −0.12V. The negative differenceindicates that the reaction is thermodynamically unfavorable.

As these examples illustrate, a reducing agent with a standard oxidationpotential between that of the oxo-anion and the halogen will reactfavorably with the oxo-anion. This ensures that the halogen generated bythe reduction of the oxo-anion will be largely available for thetreatment of kerogen, as opposed to being consumed by the reducingagent.

As discussed so far in this application, reducing agents with kineticfactors that make them more reactive to the oxo-anions than to thehalogen, or reducing agents with a standard reduction potential betweenthe oxo-anion and the halogen are suitable reducing agents. However,these are not necessary characteristics of the reducing agent, and otherreducing agents without these kinetic or thermodynamic properties mayalso be suitable for reducing oxo-anions.

A composition including oxo-anions and reducing agents can be used totreat kerogen. In some implementations, the oxo-anions are encapsulatedand the reducing agent is unencapsulated. In some implementations, thereducing agent is encapsulated and the oxo-anions are unencapsulated. Insome implementations, the oxo-anions and the reducing agent are bothencapsulated.

In some implementations, a composition to treat kerogen in asubterranean formation includes a bromate salt, a chlorate salt, aniodate salt, or mixtures thereof, where the salt or mixture of salts areencapsulated in a first polymer. The composition also includes areducing agent.

In some implementations, the first polymer is a polymer matrix, forexample a polyvinyl butyral polymer, vinyl acetal polymer, butyralpolymer, styrene/-maleic anhydride copolymer, or alkylatedvinylpyrrolidone copolymer.

In some implementations, the first polymer can be an enteric coating,for example, methylacrylate-methacrylic acid copolymers, celluloseacetate phthalate, cellulose acetate succinate, hydroxypropylmethylcellulose phthalate, hypromellose acetate succinate, polyvinyl acetatephthalate, shellac, cellulose acetate trimellitate, sodium alginate, orzein.

In some implementations, the reducing agent can have a standardreduction potential between that of the bromate or chlorate salt and thecorresponding halogen. For bromate salts, the corresponding halogen isbromine. For chlorate salts, the corresponding halogen is chlorine. Foriodate salts, the corresponding halogen is iodine.

In some implementations, the reducing agent can be sulfur, redphosphorus, bisulfate, iodide, or iodine.

In some implementations, the reducing agent is encapsulated in a secondpolymer. The second polymer can be a polymer matrix, for example apolyvinyl butyral polymer, vinyl acetal polymer, butyral polymer,styrene/-maleic anhydride copolymer, or alkylated vinylpyrrolidonecopolymer. In some implementations, the second polymer can be an entericcoating, for example methyacrylate-methacrylic acid copolymer, celluloseacetate phthalate, cellulose acetate succinate, hydroxypropylmethylcellulose phthalate, hypromellose acetate succinate, polyvinyl acetatephthalate, shellac, cellulose acetate trimellitate, sodium alginate, orzein.

FIG. 4 shows an example of a method for treating kerogen in asubterranean zone 400 includes encapsulating the at least one of abromate, chlorate, or iodate salt in a first polymer. At 402, theencapsulated salt or salts are flowed into a subterranean formationwhere kerogen is present. At 406, a reducing agent is flowed into thesubterranean formation where kerogen is present. At 408, the salt orsalts are then in contact with the reducing agent.

In the above described compositions and methods, the halogen is notgenerated until the salt or salts are in contact with the reducingagent. This happens in the subterranean formation, and therefore thereactive halogen is not generated prematurely, such as above ground orin transit to the subterranean formation. This results in increasedsafety and prevents the halogen from degrading drilling equipment.

Another approach for generating a halogen in situ is the use ofN-halosuccinimides, for example N-bromosuccinimide, N-chlorosuccinimide,or N-iodosuccinimide. These compounds are soluble in organic solvents aswell as in water. Homolytic or heterolytic fissions of these compoundsresults in a halogen radical or halogen ion. For example, the homolyticfission of N-bromosuccinimide results in a bromine radical (FIG. 5A).The heterolytic fission of N-bromosuccinimide results in bromonium (FIG.5B). Fission can occur upon exposure of the N-halosuccinimides to heator light. In some implementations, heat from the subterranean formationcan initiate the reaction. Radical initiators such as benzoyl peroxidecan also be used. The halogen radical or halonium can react with anddegrade kerogen.

N-halosuccinimides are solids at room temperature, stable, and easy tohandle. N-halosuccinimides can be dissolved in an aqueous or organicsolvents prior to being injected into a well. The stability prior tosolubilization and the solubility in both aqueous and organic solventsmake these compounds safe and practical means for generating halogens totreat kerogen.

In some implementations, a composition for treating kerogen in asubterranean formation includes at least one N-halosuccinimide dissolvedin a fracturing fluid. The concentration of the N-halosuccinimde in thefracturing fluid can be between 0.001 M and 0.10 M. In someimplementations, the fracturing fluid is an aqueous-based fluid. In someimplementations, the fracturing fluid is a carbon dioxide-based fluid orfoam.

In some implementations, an N-halosuccinimide can be encapsulated.Encapsulation of the N-halosuccinimide delays its reaction withsubstrate. Once the radical or halonium forms, it can react quickly withavailable reducing agents. Encapsulation can delay the reaction of theradical or halonium until it has reached kerogen in the formation. Inaddition, other available reducing agents that are inherent in theformation could potentially react with the radical or halonium beforethey reach the kerogen. In some implementations, the N-halosuccinimideis encapsulated in a polymer and dissolved in a carbon dioxide-basedfluid or foam. The polymer can be a polymer matrix, for example apolyvinyl butyral polymer, vinyl acetal polymer, butyral polymer,styrene/-maleic anhydride copolymer, or alkylated vinylpyrrolidonecopolymer. In some implementations, the polymer can be an entericcoating, for example methylacrylate methacrylic acid copolymer,cellulose acetate phthalate, cellulose acetate succinate,hydroxypropylmethyl cellulose phthalate, hypromellose acetate succinate,polyvinyl acetate phthalate, shellac, cellulose acetate trimellitate,sodium alginate, or zein.

The encapsulated N-halosuccinimide can be dissolved in a carbon-dioxidebased fluid at a concentration of 5 to 100 pounds of encapsulatedN-halosuccinimide per 1000 gallons of carbon dioxide-based fluid.

FIG. 6 shows an example of a method 600 for treating kerogen in asubterranean formation. At 602, at least one N-halosuccinimide from agroup consisting of N-bromosuccinimide, N-chlorosuccinimide, andN-iodosuccinimide is dissolved in a fracturing fluid to create anN-halosuccinimide solution. At 604, the N-halosuccinimide solution isflowed into a subterranean formation at which kerogen is present.

As N-halosuccinimides are stable chemicals, the generation of thehalogen radicals or ions is delayed until they reach the subterraneanzone and are therefore safer to use in the field than native halogens.

EXAMPLES Example 1: Reaction of Bromine with Kerogen in a Water-BasedMedium

Kerogen samples can be segregated into samples with relatively lowpyrite concentration and relatively high concentration using densityliquid separation, for example with zinc bromide. Kerogen samples withrelatively low pyrite concentration float (<1.8 g/cc) whereas samplewith relatively higher pyrite concentration sink (>1.8 g/cc). A piece offloated, thermally mature kerogen weighing 101.9 mg was suspended in 25mL of de-ionized water in a glass pressure tube. Next, 0.17 mL (3.3mmol) of Bra was added to the mixture, and the tube was sealed andheated to 150° C. in an oil bath for 3 hours. The mixture was thenfiltered and the isolated kerogen collected, dried, and weighed. Themass of the kerogen before treatment was 99.1 mg. The mass of thekerogen after treatment was 122.9 mg. Therefore, there was a greaterthat 20% increase in mass that can be attributed to the reaction ofbromine with the kerogen sample.

Example 2: Reaction of Bromine in Supercritical CO₂ with Shale RockSamples #1-3

Shale rock samples #1-3, each with the mineralogical composition givenin Table 1, were cut into 1 cm×1 cm×1.5 cm rectangular prisms and broadion-beam polished to afford a flack surface. The samples were thenindividually placed in a 750 mL high-pressure autoclave composed ofcorrosion resistant metal alloy. Next, 3 mL of bromine (60 mmol) wasadded to the samples and the remainder of the autoclave was filled withliquid CO₂ at 800 psi. The autoclave was then sealed and heated to 150°C. for 20 hours at a pressure of 2600 psi before being allowed to cool.Scanning electron microscopy (SEM) images of each shale sample beforeand after treatment were obtained. The first shale sample is shownbefore treatment in FIG. 7A. The first shale sample is shown aftertreatment in FIG. 7B. The second shale sample is shown before treatmentin FIG. 7C. The second shale sample is shown after treatment in FIG. 7D.The third shale sample is shown before treatment in FIG. 7E. The thirdshale sample is shown after treatment in FIG. 7F. After treatment withbromine, “eruptions” of brominated kerogen are observed in the shalerock samples, representing partially depolymerized kerogen (FIGS. 8A-C).

TABLE 1 Mineralogy of polish shale sample before treatment Sample 1Sample 2 Sample 3 Quartz 30 72  25 Albite 9 5 4 Orthoclase 2 1 8Chlorite 9 1 2 Illite/Mica 35 15  34 Illite/Smectite 7 4 14 Pyrite Trace1 10 Anatase 2 Trace 0.4 Siderite 5 0 0 Kaolinite 1 Trace 5 Gypsum 0Trace 0 Dolomite 0 1 0

The term “about” as used in this disclosure can allow for a degree ofvariability in a value or range, for example, within 10%, within 5%, orwithin 1% of a stated value or of a stated limit of a range.

The term “substantially” as used in this disclosure refers to a majorityof, or mostly, as in at least about 50%, 60%, 70%, 80%, 90%, 95%, 96%,97%, 98%, 99%, 99.5%, 99.9%, 99.99%, or at least about 99.999% or more.

The term “solvent” as used in this disclosure refers to a liquid thatcan dissolve a solid, another liquid, or a gas to form a solution.Non-limiting examples of solvents are silicones, organic compounds,water, alcohols, ionic liquids, and supercritical fluids.

The term “room temperature” as used in this disclosure refers to atemperature of about 15 degrees Celsius (° C.) to about 28° C.

The term “downhole” as used in this disclosure refers to under thesurface of the earth, such as a location within or fluidly connected toa wellbore.

As used in this disclosure, the term “fracturing fluid” refers to fluidsor slurries used downhole during fracturing operations.

As used in this disclosure, the term “fluid” refers to liquids and gels,unless otherwise indicated.

As used in this disclosure, the term “subterranean material” or“subterranean zone” refers to any material under the surface of theearth, including under the surface of the bottom of the ocean. Forexample, a subterranean zone or material can be any section of awellbore and any section of a subterranean petroleum- or water-producingformation or region in fluid contact with the wellbore. Placing amaterial in a subterranean zone can include contacting the material withany section of a wellbore or with any subterranean region in fluidcontact the material. Subterranean materials can include any materialsplaced into the wellbore such as cement, drill shafts, liners, tubing,casing, or screens; placing a material in a subterranean zone caninclude contacting with such subterranean materials. In some examples, asubterranean zone or material can be any downhole region that canproduce liquid or gaseous petroleum materials, water, or any downholesection in fluid contact with liquid or gaseous petroleum materials, orwater. For example, a subterranean zone or material can be at least oneof an area desired to be fractured, a fracture or an area surrounding afracture, and a flow pathway or an area surrounding a flow pathway, inwhich a fracture or a flow pathway can be optionally fluidly connectedto a subterranean petroleum- or water-producing region, directly orthrough one or more fractures or flow pathways.

As used in this disclosure, “treatment of a subterranean zone” caninclude any activity directed to extraction of water or petroleummaterials from a subterranean petroleum- or water-producing formation orregion, for example, including drilling, stimulation, hydraulicfracturing, clean-up, acidizing, completion, cementing, remedialtreatment, abandonment, aquifer remediation, identifying oil richregions via imaging techniques, and the like.

As used in this disclosure, a “flow pathway” downhole can include anysuitable subterranean flow pathway through which two subterraneanlocations are in fluid connection. The flow pathway can be sufficientfor petroleum or water to flow from one subterranean location to thewellbore or vice-versa. A flow pathway can include at least one of ahydraulic fracture, and a fluid connection across a screen, acrossgravel pack, across proppant, including across resin-bonded proppant orproppant deposited in a fracture, and across sand. A flow pathway caninclude a natural subterranean passageway through which fluids can flow.In some implementations, a flow pathway can be a water source and caninclude water. In some implementations, a flow pathway can be apetroleum source and can include petroleum. In some implementations, aflow pathway can be sufficient to divert water, a downhole fluid, or aproduced hydrocarbon from a wellbore, fracture, or flow pathwayconnected to the pathway.

As used in this disclosure, “weight percent” (wt %) can be considered amass fraction or a mass ratio of a substance to the total mixture orcomposition. Weight percent can be a weight-to-weight ratio ormass-to-mass ratio, unless indicated otherwise.

A number of implementations of the disclosure have been described.Nevertheless, it will be understood that various modifications may bemade without departing from the spirit and scope of the disclosure.

What is claimed is:
 1. A composition for treating kerogen in asubterranean formation, comprising: at least one of a bromate orchlorate salt, wherein the bromate or chlorate salt is encapsulated in afirst polymer; and an acid.
 2. The composition of claim 1, wherein thefirst polymer is a polymer matrix selected from a group consisting ofpolyvinyl butyral polymer, vinyl acetal polymers, butyral polymers,sytrene/-maleic anhydride copolymers, and alkylated vinylpyrrolidonecopolymers.
 3. The composition of claim 1, wherein the first polymer isan enteric coating and is selected from a group consisting ofmethylacrylate-methacrylic acid copolymers, cellulose acetate phthalate,cellulose acetate succinate, hydroxypropylmethyl cellulose phthalate,hypromellose acetate succinate, polyvinyl acetate phthalate, shellac,cellulose acetate trimellitate, sodium alginate, and zein.
 4. Thecomposition of claim 1, wherein the acid is encapsulated in a secondpolymer, wherein the second polymer is a polymer matrix selected from agroup consisting of polyvinyl butyral polymer, vinyl acetal polymers,butyral polymers, sytrene/-maleic anhydride copolymers, and alkylatedvinylpyrrolidone copolymers.
 5. The composition of claim 1, wherein theacid is encapsulated in a second polymer, wherein the second polymer isan enteric coating selected from a group consisting ofmethylacrylate-methacrylic acid copolymers, cellulose acetate phthalate,cellulose acetate succinate, hydroxypropylmethyl cellulose phthalate,hypromellose acetate succinate, polyvinyl acetate phthalate, shellac,cellulose acetate trimellitate, sodium alginate, and zein.
 6. Thecomposition of claim 1, wherein the acid is lactic acid, polylacticacid, chloracetic acid, citric acid, oxalic acid, benzoic acid, furoicacid, or aqueous hydrochloric acid.
 7. A method for treating kerogen ina subterranean formation, comprising: encapsulating at least one of abromate or chlorate salt in a first polymer; flowing the encapsulatedbromate or chlorate salt into a subterranean formation at which kerogenis present; selecting an acid; flowing the acid into a location in thesubterranean formation at which kerogen is present; and contacting thebromate or chlorate salt with the acid.
 8. The method of claim 7,wherein encapsulating at least one of a bromate or chlorate salt in afirst polymer comprises encapsulating the bromate or chlorate salt in apolymer matrix selected from a group consisting of polyvinyl butyralpolymer, vinyl acetal polymers, butyral polymers, sytrene/-maleicanhydride copolymers, and alkylated vinylpyrrolidone copolymers.
 9. Themethod of claim 7, wherein encapsulating at least one of a bromate orchlorate salt in a first polymer comprises encapsulating the bromate orchlorate salt in an enteric coating selected from a group consisting ofmethylacrylate-methacrylic acid copolymers, cellulose acetate phthalate,cellulose acetate succinate, hydroxypropylmethyl cellulose phthalate,hypromellose acetate succinate, polyvinyl acetate phthalate, shellac,cellulose acetate trimellitate, sodium alginate, and zein.
 10. Themethod of claim 7, wherein selecting the acid comprises selecting theacid from a group consisting of lactic acid, polylactic acid,chloracetic acid, citric acid, oxalic acid, benzoic acid, furoic acid,and aqueous hydrochloric acid.
 11. The method of claim 7, furthercomprising encapsulating the selected acid in a second polymer, whereinthe second polymer is a polymer matrix selected from a group consistingof polyvinyl butyral polymer, vinyl acetal polymers, butyral polymers,sytrene/-maleic anhydride copolymers, and alkylated vinylpyrrolidonecopolymers.
 12. The method of claim 7, further comprising encapsulatingthe selected acid in a second polymer, wherein the second polymer is anenteric coating selected from a group consisting ofmethylacrylate-methacrylic acid copolymers, cellulose acetate phthalate,cellulose acetate succinate, hydroxypropylmethyl cellulose phthalate,hypromellose acetate succinate, polyvinyl acetate phthalate, shellac,cellulose acetate trimellitate, sodium alginate, and zein.
 13. Themethod of claim 7, further comprising simultaneously flowing the saltand the acid into the subterranean formation.